1. Field of the Invention
The present invention relates to an electromagnetic actuator having a two-part actuating element.
2. Description of the Related Art
In modern passenger-car automatic transmissions, hydraulically actuated clutches are used for changing gears. In order for the shifting operations to proceed smoothly and imperceptibly for the driver, it is necessary to adjust the hydraulic pressure on the clutches in accordance with predefined pressure ramps with high pressure precision. The pressure required for this purpose may be provided with the aid of hydraulic slide valves, as they are known for example from published international patent application publication WO 2011/012366 A1. These slide valves may be actuated either via a pilot valve or directly via an electromagnetic actuator. Electromagnetic actuators may moreover also be used for actuating shift valves, developed as seated valves, in automatic transmissions or other applications. The flawless functioning and installation of electromagnetic actuators in an automatic transmission requires special measures. Often, for example, a force transmission element is required for actuating a slide valve, which is normally called an actuating element or actuating pin.
Published German patent application document DE 10 2006 011 078 A1 describes an electromagnetic actuator, which is called a solenoid in this document, which has a magnetic coil, which has a coil form having a coil winding situated on it and a central clearance extending in an axial direction, in which an armature is situated sliding in a bearing sleeve. On an axial end of the coil form, a terminal piece covering the central clearance is developed, which is made up of a cone and a cone disk. An actuating rod is supported in a sliding manner in the terminal piece, the armature being movable by an actuation of the magnetic coil such that a force applied by the armature of the electromagnetic actuator is transmittable to the actuating. rod. In the known electromagnetic actuator, the actuating rod is connected in a fixed manner to the armature by a press-fit connection.
The actuating rod known from Published German patent application document DE 10 2006 011 078 A1 is fitted with a collar that is produced by an elaborate cutting process. Since the collar is not formed while the actuating rod is mounted on the armature, but must be produced ahead of time by cutting, it is not possible flexibly to adapt during installation the length of the actuating rod that is protruding from the armature to the individual conditions. The collar acts at the same time also to prevent a magnetic bonding in that, by situating the collar between the pole of the armature and an opposite pole of the terminal piece, the surfaces of the two poles are prevented from touching each other. Since the collar for this purpose must be made from non-magnetic material, the entire actuating rod must also be made from non-magnetic material, which has the consequence that the material of the actuating rod cannot be selected in optimized fashion with respect to force transmission capacity and wear with regard to stability and hardness. Moreover, the actuating element made from a non-magnetic material has a greater thermal expansion than the terminal piece made from magnetic material, in which the actuating rod is supported in a bore in sliding fashion. This has the consequence that with rising temperature the running clearance required by the actuating rod is reduced and particles present in a gap between the bore and the actuating rod may cause the actuating rod to seize.
Since the collar furthermore has only a very small elasticity of its own, the impulse forces occurring when there is impact stress become very high. In order to avoid wear, the actuating rod must therefore be designed as having a relatively large diameter in order to keep the occurring surface pressures low. Due to the size of the actuating rod, the actuating rod is relatively expensive and causes a considerable pump effect when the armature moves in the bore, which may transport contaminated oil into the magnet.
Because of the fixed connection between the armature and the actuating rod, the sleeve bearing of the actuating rod must additionally absorb magnetic transverse forces, which results in an increased magnetic force hysteresis. In the known design approach there is additionally the danger that the manufacturing-related eccentricity between the bearing points on the armature and on the actuating rod may cause jamming. In order to prevent the moving parts from jamming, the sleeve bearing of the actuating rod must be designed to be relatively short so as to allow for inclinations, which, if necessary, are able to compensate for a manufacturing-related eccentricity. The short bearing, however, has the disadvantage that dirt-sealing effect is clearly reduced.
The objective of the present invention is to provide an electromagnetic actuator that avoids the disadvantages described above.